Heretofore, generally, in addition to iron-based materials, copper-based materials such as phosphor bronze, red brass, and brass, which are excellent in electrical conductivity and thermal conductivity, have been used widely as materials for parts of electric and electronic machinery and tools (electrical and electronic instruments).
Recently, demands for miniaturization, weight saving, and associated high-density packaging of parts of electric and electronic machinery and tools have increased, and various characteristics are required for the copper-based materials applied thereto. Examples of basic characteristics required include mechanical properties, electrical conductivity, stress relaxation resistance, and bending property. Of those, improvements in tensile strength and bending property are strongly required, for satisfying the recent demands for the miniaturization of parts or components for the products described above.
The requirements depend on shapes or the like of the parts, and specific requirements include: a tensile strength of 720 MPa or more and a bending property of R/t≦1 (in which R represents a bending radius, and t represents a thickness); a tensile strength of 800 MPa or more and a bending property of R/t<1.5; or a tensile strength of 900 MPa or more and a bending property of R/t<2. The required characteristics have reached a level that cannot be satisfied with conventional commercially available, mass-produced alloys such as phosphor bronze, red brass, and brass. Such alloys each have an increased strength by: allowing Sn or Zn having a very different atomic radius from that of copper as a matrix phase, to be contained as a solid solution in Cu; and subjecting the resultant alloy having the solid solution to cold working such as rolling or drawing. The method can provide high-strength materials, by employing a large cold working ratio, but employment of a large cold working ratio (generally 50% or more) is known to conspicuously degrade bending property of the resultant alloy material. The method generally involves a combination of solid solution strengthening and working strengthening.
An alternative strengthening method is a precipitation strengthening method that involves formation of a precipitate of a nanometer order in the materials. The precipitation strengthening method has merits of increasing strength and improving electrical conductivity at the same time, and is used for many alloys.
Of those, a strengthened alloy prepared by forming a precipitate composed of Ni and Si by adding Ni and Si into Cu, so-called a Corson alloy, has a remarkably high strengthening ability compared with many other precipitation-type alloys (precipitation hardened alloys). This strengthening method is also used for some commercially available alloys (e.g. CDA70250, a registered alloy of Copper Development Association (CDA)). When the alloy generally subjected to precipitating strengthening is used for terminal/connector materials, the alloy is produced through a production process incorporating the following two important heat treatments. A first heat treatment involves heat treatment at a high temperature (generally 700° C. or higher) near a melting point, so-called solution treatment, to allow Ni and Si precipitated through casting or hot rolling to be contained as a solid solution into a Cu matrix. A second heat treatment involves heat treatment at a lower temperature than that of the solution treatment, so-called aging treatment, to precipitate Ni and Si, which are in the solid solution caused at the high temperature, as a precipitate. The strengthening method utilizes a difference in concentrations of Ni and Si entering Cu as a solid solution at high temperatures and low temperatures, and the method itself is a well-known technique in production of precipitation-type alloys.
An example of the Corson alloy suitable for parts of electric and electronic machinery and tools includes an alloy having a defined crystal grain size.
However, the precipitation-type alloy has such problems that the crystal grain size increases to cause too large crystal grains during the solution treatment, and that the crystal grain size during the solution treatment remains unchanged and becomes the crystal grain size of a product since the aging treatment generally does not involve recrystallization. An increased amount of Ni or Si to be added requires a solution treatment at a higher temperature, and it results in that the crystal grain size tends to increase to cause too large crystal grains, through a heat treatment in a short period of time. Too large crystal grains occurred in this way cause problems of conspicuous deterioration in bending property.
Alternatively, a method of improving the bending property of a copper alloy involves addition of Mn, Ni, and P for a mutual reaction to precipitate a compound, without use of a Ni—Si precipitate.
However, the alloy has a tensile strength of about 640 MPa at most, which is not sufficient for satisfying the recent demands for high strength through miniaturization of parts. Addition of Si to the copper alloy decreases the amount of the Ni—P precipitate, to thereby reduce the mechanical strength and electrical conductivity. Further, excess Si and P cause problems of occurrence of crack during hot working.
The bending property is hardly maintained with increasing tensile strength, and a copper alloy having tensile strength, bending property, and electrical conductivity at high levels has been required.
Other and further features and advantages of the invention will appear more fully from the following description.